|Publication number||US7433993 B2|
|Application number||US 10/749,189|
|Publication date||Oct 7, 2008|
|Filing date||Dec 30, 2003|
|Priority date||Dec 30, 2003|
|Also published as||US20050144357|
|Publication number||10749189, 749189, US 7433993 B2, US 7433993B2, US-B2-7433993, US7433993 B2, US7433993B2|
|Inventors||Alan Welsh Sinclair|
|Original Assignee||San Disk Corportion|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (83), Non-Patent Citations (15), Referenced by (33), Classifications (10), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This invention relates generally to the operation of non-volatile memory systems, and, more specifically, to the handling of data within such memory systems.
There are many commercially successful non-volatile memory products being used today, particularly in the form of small form factor cards, which employ an array of flash EEPROM (Electrically Erasable and Programmable Read Only Memory) cells formed on one or more integrated circuit chips. A memory controller, usually but not necessarily on a separate integrated circuit chip, interfaces with a host to which the card is removably connected and controls operation of the memory array within the card. Such a controller typically includes a microprocessor, some non-volatile read-only-memory (ROM), a volatile random-access-memory (RAM) and one or more special circuits such as one that calculates an error-correction-code (ECC) from data as they pass through the controller during the programming and reading of data. Some of the commercially available cards are CompactFlash™ (CF) cards, MultiMedia cards (MMC), Secure-Digital (SD) cards, Smart Media cards, personnel tags (P-Tag) and Memory Stick cards. Hosts include personal computers, notebook computers, personal digital assistants (PDAs), various data communication devices, digital cameras, cellular telephones, portable audio players, automobile sound systems, and similar types of equipment. Besides the memory card implementation, this type of memory can alternatively be embedded into various types of host systems.
Two general memory cell array architectures have found commercial application, NOR and NAND. In a typical NOR array, memory cells are connected between adjacent bit line source and drain diffusions that extend in a column direction with control gates connected to word lines extending along rows of cells. A memory cell includes at least one storage element positioned over at least a portion of the cell channel region between the source and drain. A programmed level of charge on the storage elements thus controls an operating characteristic of the cells, which can then be read by applying appropriate voltages to the addressed memory cells. Examples of such cells, their uses in memory systems and methods of manufacturing them are given in U.S. Pat. Nos. 5,070,032, 5,095,344, 5,313,421, 5,315,541, 5,343,063, 5,661,053 and 6,222,762.
The NAND array utilizes series strings of more than two memory cells, such as 16 or 32, connected along with one or more select transistors between individual bit lines and a reference potential to form columns of cells. Word lines extend across cells within a large number of these columns. An individual cell within a column is read and verified during programming by causing the remaining cells in the string to be turned on hard so that the current flowing through a string is dependent upon the level of charge stored in the addressed cell. Examples of NAND architecture arrays and their operation as part of a memory system are found in U.S. Pat. Nos. 5,570,315, 5,774,397, 6,046,935, and 6,522,580.
The charge storage elements of current flash EEPROM arrays, as discussed in the foregoing referenced patents, are most commonly electrically conductive floating gates, typically formed from conductively doped polysilicon material. An alternate type of memory cell useful in flash EEPROM systems utilizes a non-conductive dielectric material in place of the conductive floating gate to store charge in a non-volatile manner. A triple layer dielectric formed of silicon oxide, silicon nitride and silicon oxide (ONO) is sandwiched between a conductive control gate and a surface of a semi-conductive substrate above the memory cell channel. The cell is programmed by injecting electrons from the cell channel into the nitride, where they are trapped and stored in a limited region, and erased by injecting hot holes into the nitride. Several specific cell structures and arrays employing dielectric storage elements are described in United States patent application publication no. 2003/0109093 of Harari et al.
Individual flash EEPROM cells store an amount of charge in a charge storage element or unit that is representative of one or more bits of data. The charge level of a storage element controls the threshold voltage (commonly referenced as VT) of its memory cell, which is used as a basis of reading the storage state of the cell. A threshold voltage window is commonly divided into a number of ranges, one for each of the two or more storage states of the memory cell. These ranges are separated by guardbands that include a nominal sensing level that allows determining the storage states of the individual cells. These storage levels do shift as a result of charge disturbing programming, reading or erasing operations performed in neighboring or other related memory cells, pages or blocks. Error correcting codes (ECCs) are therefore typically calculated by the controller and stored along with the host data being programmed and used during reading to verify the data and perform some level of data correction if necessary. Also, shifting charge levels can be restored back to the centers of their state ranges from time-to-time, before disturbing operations cause them to shift completely out of their defined ranges and thus cause erroneous data to be read. This process, termed data refresh or scrub, is described in U.S. Pat. Nos. 5,532,962 and 5,909,449.
As in most all integrated circuit applications, the pressure to shrink the silicon substrate area required to implement some integrated circuit function also exists with flash EEPROM memory cell arrays. It is continually desired to increase the amount of digital data that can be stored in a given area of a silicon substrate, in order to increase the storage capacity of a given size memory card and other types of packages, or to both increase capacity and decrease size. One way to increase the storage density of data is to store more than one bit of data per memory cell and/or per storage, unit or element. This is accomplished by dividing a window of a storage element charge level voltage range into more than two states. The use of four such states allows each cell to store two bits of data, eight states stores three bits of data per storage element, and so on. Multiple state flash EEPROM structures using floating gates and their operation are described in U.S. Pat. Nos. 5,043,940 and 5,172,338, and for structures using dielectric floating gates in aforementioned United States patent application publication no. 2003/0109093. Selected portions of a multi-state memory cell array may also be operated in two states (binary) for various reasons, in a manner described in U.S. Pat. Nos. 5,930,167 and 6,456,528.
Memory cells of a typical flash EEPROM array are divided into discrete blocks of cells that are erased together. That is, the erase block is the erase unit, a minimum number of cells that are simultaneously erasable. Each erase block typically stores one or more pages of data, the page being the minimum unit of programming and reading, although more than one page may be programmed or read in parallel in different sub-arrays or planes. Each page typically stores one or more sectors of data, the size of the sector being defined by the host system. An example sector includes 512 bytes of user data, following a standard established with magnetic disk drives, plus some number of bytes of overhead information about the user data and/or the erase block in which they are stored. Such memories are typically configured with 16, 32 or more pages within each erase block, and each page stores one or just a few host sectors of data.
In order to increase the degree of parallelism during programming user data into the memory array and read user data from it, the array is typically divided into sub-arrays, commonly referred to as planes, which contain their own data registers and other circuits to allow parallel operation such that sectors of data may be programmed to or read from each of several or all the planes simultaneously. An array on a single integrated circuit may be physically divided into planes, or each plane may be formed from a separate one or more integrated circuit chips. Examples of such a memory implementation are described in U.S. Pat. Nos. 5,798,968 and 5,890,192.
In some memory systems, the physical memory cells are also grouped into two or more zones. A zone may be any partitioned subset of the physical memory or memory system into which a specified range of logical block addresses is mapped. For example, a memory system capable of storing 64 Megabytes of data may be partitioned into four zones that store 16 Megabytes of data per zone. The range of logical block addresses is then also divided into four groups, one group being assigned to the erase blocks of each of the four zones. Logical block addresses are constrained, in a typical implementation, such that the data of each are never written outside of a single physical zone into which the logical block addresses are mapped. In a memory cell array divided into planes (sub-arrays), which each have their own addressing, programming and reading circuits, each zone preferably includes erase blocks from multiple planes, typically the same number of erase blocks from each of the planes. Zones are primarily used to simplify address management such as logical to physical translation, resulting in smaller translation tables, less RAM memory needed to hold these tables, and faster access times to address the currently active region of memory, but because of their restrictive nature can result in less than optimum wear leveling.
To further efficiently manage the memory, erase blocks may be linked together to form virtual blocks or metablocks. That is, each metablock is defined to include one erase block from each plane. Use of the metablock is described in international patent application publication no. WO 02/058074. The metablock is identified by a host logical block address as a destination for programming and reading data. Similarly, all erase blocks of a metablock are erased together. The controller in a memory system operated with such large blocks and/or metablocks performs a number of functions including the translation between logical block addresses (LBAs) received from a host, and physical block numbers (PBNs) within the memory cell array. Individual pages within the blocks are typically identified by offsets within the block address. Address translation often involves use of intermediate terms of a logical block number (LBN) and logical page.
Data stored in a metablock are often updated, the likelihood of updates occurring in a metablock increases as the data capacity of the metablock increases. Updated sectors of one metablock are normally written to another metablock. The unchanged sectors are usually also copied from the original to the new metablock, as part of the same programming operation, to consolidate the data. Alternatively, the unchanged data may remain in the original metablock until later consolidation with the updated data into a single metablock again.
Copying unchanged sectors may add to the time required for copying and adds to the space occupied by the data in the memory array because the original metablock may not be used until an erase operation is performed. Copying of unchanged sectors is a result of logical fragmentation of host files into different metablocks. Where a metablock contains portions of two host files, updating one of the files also involves copying the portion of the other file that is stored in the same metablock. As metablocks become larger, the portions being copied also become larger. Thus, logical fragmentation becomes a greater problem as metablocks become larger.
It is common to operate large block or metablock systems with some extra erase blocks maintained in an erased block pool. When one or more pages of data less than the capacity of an erase block are being updated, it is typical to write the updated pages to an erase block from the pool and then copy data of the unchanged pages from the original erase block to erase pool block. Variations of this technique are described in aforementioned published international application no. WO 02/058074. Over time, as a result of host data files being re-written and updated, many erase blocks can end up with a relatively few number of its pages containing valid data and remaining pages containing data that is no longer current. In order to be able to efficiently use the data storage capacity of the array, logically related data pages of valid data are from time-to-time gathered together from fragments among multiple. erase blocks and consolidated together into a fewer number of erase blocks. This process is commonly termed “garbage collection.”
Data may be stored in a memory array in adaptive metablocks. The size of an adaptive metablock may be tailored to the data to be stored. Adaptive metablock size may be determined based on the nature of the data (control data, data from host) or may be determined based on boundaries within the data, such as boundaries between files. Configuring adaptive metablocks according to the data reduces the effects logical fragmentation.
Logical groups that contain data equal to the data in one erase block of a memory array are formed from logically sequential sectors. Adaptive logical blocks are formed from logical groups. Adaptive logical blocks may contain different numbers of logical groups. Individual adaptive logical blocks are stored in individual adaptive metablocks in a memory array. The number of erase blocks in an adaptive metablock is equal to the number of logical groups in the corresponding adaptive logical block. Thus, an adaptive metablock has a variable number of erase blocks. The erase blocks of a metablock may be from fewer than all the planes of the memory array. More than one adaptive metablock may be programmed at one time. Adaptive metablocks may be formed according to the data to be stored. Large adaptive metablocks may be used to attain a high degree of parallelism during programming. Smaller adaptive metablocks may be used to allow efficient updating of stored data.
Adaptive logical blocks may be formed so that boundaries between adaptive logical blocks reflect boundaries in the data, for example boundaries between files or streams of data. By tailoring adaptive logical blocks in this way, copying of data within the memory array may be reduced. Where data is updated, a new adaptive logical block may be formed to hold the updated data with a small amount of old data. Thus, if the same data is updated again, there is only a small amount of old data that needs to be copied.
Where an adaptive logical block is partially filled, the data may be copied to a smaller adaptive logical block. This may be done before the partially filled adaptive logical block is programmed or it may be done after the partially filled adaptive logical block is programmed in an adaptive metablock, in which case the adaptive metablock containing the partially filled adaptive logical block is marked as obsolete. The smaller adaptive logical block is programmed to a smaller adaptive metablock in the memory array. Thus, there is a saving of space in the memory array.
In architectures that use non-sequentially updated metablocks (chaotic blocks) to hold update data, an adaptive metablock may be used instead. The size of the adaptive metablock may be selected according to the logical address range that is being updated. If the adaptive metablock is tailored to a particular logical address range, updates in that range may be performed more efficiently because there is less copying of data.
Formation of adaptive metablocks and recording the location of stored data is performed by a media manager. A media manager maintains records of available erase blocks. Records of locations of stored data are also maintained by the media manager. Records of locations of stored data are maintained in tables (or lists) have an entry for each logical group. The entry for each logical group indicates the size of the adaptive metablock (and corresponding adaptive logical block) containing the logical group, the position of the logical group within its adaptive logical block and the physical location of one of the erase blocks of the metablock.
Memory Architectures and Their Operation
Referring initially to
A typical controller 19 includes a microprocessor 21, a read-only-memory (ROM) 23 primarily to store firmware and a buffer memory (RAM) 25 primarily for the temporary storage of user data either being written to or read from the memory chips 11 and 13. Buffer memory 25 may be either volatile or non-volatile memory. Circuits 27 interface with the memory array chip(s) and circuits 29 interface with a host though connections 31. The integrity of data is in this example determined by calculating an ECC with circuits 33 dedicated to calculating the code. As user data is being transferred from the host to the flash memory array for storage, the circuit calculates an ECC from the data and the code is stored in the memory. When that user data are later read from the memory, they are again passed through the circuit 33, which calculates the ECC by the same algorithm and compares that code with the one calculated and stored with the data. If they compare, the integrity of the data is confirmed. If they differ, depending upon the specific ECC algorithm utilized, those bits in error, up to a number supported by the algorithm, can be identified and corrected.
The connections 31 of the memory of
The memory of
The size of the individual memory cell erase blocks of
The parameters 55 may include a quantity related to the number of program/erase cycles experienced by the erase block, this quantity being updated after each cycle or some number of cycles. When this experience quantity is used in a wear leveling algorithm, logical block addresses are regularly re-mapped to different physical block addresses in order to even out the usage (wear) of all the erase blocks. Another use of the experience quantity is to change voltages and other parameters of programming, reading and/or erasing as a function of the number of cycles experienced by different erase blocks.
The parameters 55 may also include an indication of the bit values assigned to each of the storage states of the memory cells, referred to as their “rotation”. This also has a beneficial effect in wear leveling. One or more flags may also be included in the parameters 55 that indicate status or states. Indications of voltage levels to be used for programming and/or erasing the erase block can also be stored within the parameters 55, these voltages being updated as the number of cycles experienced by the erase block and other factors change. Other examples of the parameters 55 include an identification of any defective cells within the erase block, the logical address of the data that is mapped into this physical block and the address of any substitute erase block in case the primary erase block is defective. The particular combination of parameters 55 that are used in any memory system will vary in accordance with the design. Also, some or all of the overhead data can be stored in erase blocks dedicated to such a function, rather than in the erase block containing the user data or to which the overhead data pertains.
Different from the single data sector erase block of
Re-writing the data of an entire erase block usually involves programming the new data into an available erase block of an erase block pool, the original erase block then being erased and placed in the erase pool. When data of less than all the pages of an erase block are updated, the updated data are typically stored in a page of an erase block from the erased block pool and data in the remaining unchanged pages are copied from the original erase block into the new erase block. The original erase block is then erased. Variations of this large block management technique include writing the updated data into a page of another erase block without moving data from the original erase block or erasing it. This results in multiple pages having the same logical address. The most recent page of data is identified by some convenient technique such as the time of programming that is recorded as a field in sector or page overhead data.
A further multi-sector erase block arrangement is illustrated in
Yet another memory cell arrangement is illustrated in
Logical groups are formed into adaptive logical blocks. Adaptive logical blocks or logical blocks may also be referred to as “metagroups.” The term “metagroup” is considered equivalent to the term “adaptive logical block.” The term “adaptive logical block” is generally used in this application. An adaptive logical block contains a variable number of logical groups. Thus, in
In some examples of metablock architecture, metablock size is fixed. The number of planes in an array may determine the size of the metablock. In these examples, the size of logical blocks is also fixed and sectors are mapped to logical blocks in a predetermined fashion. Thus, the logical address space is divided into equal sized logical blocks having fixed logical address ranges and fixed boundary locations. In contrast, in architectures using adaptive metablocks, adaptive logical blocks do not have fixed sizes and adaptive logical blocks are not limited to predetermined ranges of logical address space. Instead, adaptive logical blocks may be of various sizes and may be formed to extend over different ranges of logical address space. The formation of logical groups facilitates adaptive metablock architecture by providing an intermediate data unit from which adaptive logical blocks of various sizes may be formed. Thus, an adaptive metablock is an example of a metablock that does not have fixed size and an adaptive logical block is an example of a logical block that does not have fixed size.
The planes used to form an adaptive metablock may be selected according to an algorithm that provides efficient use of the erase blocks of the array. Planes may be given different priority based on the number of available erase blocks in a plane and whether a particular plane is still busy from a previous operation. Also, consideration may be given to using the same planes for new material as is used for the material that is being updated so that a copy operation may be performed within the plane. Such copying of data within a plane (on-chip copy) may be more efficient in some architectures. Generally, the selection of particular erase blocks within the selected planes is not critical.
One result of having adaptive metablocks of different sizes is that some adaptive metablocks may not contain an erase block from every plane of the array. If such an adaptive metablock is programmed individually then programming does not use the maximum possible parallelism. For example, in
An algorithm assigns planes according to various criteria so that adaptive logical block 1001 is programmed to erase blocks in planes 1, 2 and 5 while adaptive logical block 1002 is programmed to erase blocks in planes 0 and 4. No erase block in plane 3 is programmed in this operation. While maximum parallelism is desirable, all six planes may not be program med together in every programming operation. A plane may not be programmed if there are no erase blocks available in the plane. If very few erase blocks are available in the plane then it is assigned a low priority when planes are being selected for programming. Here, only five erase blocks are needed to store adaptive logical blocks 1001 and 1002. Therefore, only five planes are selected and plane 3 is not selected. Plane 3 is the plane with the lowest priority in this operation. However, the priority may be reassessed when the next program operation takes place. Priorities may have changed for the next operation because one more erase block in each of planes 0,1,2,4,5 has been used. Thus, plane 3 may be used in a subsequent programming operation if there are erase blocks available in plane 3. This algorithm balances the number of erase blocks used in different planes so that a particular plane does not fill up more rapidly and become unavailable.
The planes used for an individual adaptive metablock do not have to be physically adjacent. For example, an adaptive metablock 1030 of
When all data in an adaptive metablock had been superseded by updated or relocated versions of the data, and has become obsolete, the erase blocks forming the adaptive metablock should be erased. However, the adaptive metablock may not contain an erase block from every plane of the array and, when such an adaptive metablock is erased individually, erasure does not use the maximum parallelism. Maximum speed is therefore not achieved for erasing data and the effective programming speed of the memory system is therefore reduced from the maximum possible, since programming of data may not be carried out during an erase operation in flash memory chips in common use. This may be overcome by delaying erasure of erase blocks forming an adaptive metablock until one erase block from each plane is available, to achieve maximum erase parallelism. Erase blocks available for erasure are held in a list, and sets of blocks are periodically scheduled for erasure to achieve maximum possible parallelism. Erasure of a smaller set of blocks may be performed when the list contains no blocks in some planes.
At a later time, original adaptive logical blocks may be replaced with new adaptive logical blocks by remapping logical groups. For example, in the first update of
New data 1210 extends over a logical address range that is within the address range of three sequential logical groups 1241, 1242 and 1243. Each of logical groups 1241-1243 has at least some portion that is to be updated.
Copying of data from a partially full metablock to a smaller metablock may be triggered by an elapse of time from the receipt of the stream of sectors of data 1305. Copying may also be done as part of a garbage collection routine. A smaller adaptive metablock such as 1340 may be formed directly from received data if the end of the stream of sectors of data 1305 is detected while the stream of sectors of data 1305 is in a buffer. In this case, data is not first written to a larger adaptive metablock and then copied to a smaller metablock. Thus, there is no obsolete adaptive metablock to erase. In some architectures, a host may send a signal indicating where the end of the stream of data occurs. An adaptive logical block may then be formed to contain only logical groups that contain sectors from the stream of data.
In certain memory architectures, erase blocks or metablocks may be assigned for storing updated data. Examples of such erase blocks and metablocks are described in the patent application having an attorney docket number SNDK.247US0, entitled “Management of non-volatile memory systems having large erase blocks” by Conley et al, filed on the same date as the present application and hereby incorporated by reference in its entirety. Certain metablocks, designated as E1 and E2 may be used to store updated data for a plane of a memory array. Other erase blocks or metablocks, designated as dE1 may be assigned to receive updated data for a particular erase block or metablock. An adaptive metablock may be designated as E1, E2, or dE1. Such an adaptive metablock may be tailored to a logical address range that is updated frequently. By forming an adaptive metablock that has a size that is selected to fit the updated data, copying of original data may be reduced. E1 and dE1 receive update data and store them in a non-sequential manner. Update blocks (or metablocks, or adaptive metablocks) that store update data non-sequentially are considered chaotic blocks.
During the first consolidation, only the most recent copy of each sector is copied to new adaptive metablocks 1422-1424. For updated data, the most recent copy comes from adaptive metablock 1420, for data that is not updated the most recent copy comes from adaptive metablock 1410. Consolidation combines data from adaptive metablock 1410 and adaptive metablock 1420 in logical sequence. The logical address range assigned to adaptive metablock 1423 includes the logical address range of first update data 1415. Adaptive metablocks 1422, 1424 contain only data that was not updated.
Second update data 1425 are received after the first consolidation. Second update data 1425 are within the same logical address range as first update data 1415. Second update data 1425 are assigned to a new adaptive logical block 1431 that is stored in adaptive metablock 1430. Adaptive logical block 1431 has the same logical address range as data stored in adaptive metablock 1423. Adaptive metablock 1430 may be updated chaotically and so become a chaotic block. When adaptive metablock 1430 is filled, the data in adaptive metablock 1430 and adaptive metablock 1423 are consolidated to adaptive metablock 1440. Adaptive metablock, 1440 then replaces adaptive metablock 1423 and adaptive metablock 1423 may be marked as obsolete. Adaptive metablocks 1422 and 1424 remain unchanged. A smaller logical address range is consolidated in the second consolidation than in the first so that there is less copying of unchanged data. Also, less space is required in the memory array because the adaptive metablock used for updates is smaller after the first consolidation. Further updates may be made within the same logical address range and may be consolidated as in the second consolidation.
A table of the location of particular logical groups may be kept in volatile or non-volatile memory as part of media management of the memory system. A media management system may have various tables recording the location of available erase blocks and logical to physical mapping of data. A media manager manages the tables of the media management system. Typically, a media manager is implemented in firmware in a controller.
An adaptive metablock manager determines the number of logical groups to assemble to form an adaptive logical block and thus the number of erase blocks in an adaptive metablock. Where data is received from a host this determination may be based on several factors. Command sequences from the host may be evaluated and adaptive metablock size may be determined based on the current command or on historical evaluation of host commands. Characteristics of the current command that may be evaluated include logical address, command sector count, alignment with file system cluster (such as DOS cluster), logical relationship to previous command and address relative to file system sectors. The address relative to that of a range being managed by a non-sequential type of update block can also be considered. Characteristics of historical operation can include host command sequences for streams of sequential data, host command structures for complete files, records of frequently updated logical address ranges and final addresses of recently written sequential data. The adaptive metablock manager may establish a dialogue with the host, under an appropriate host interface protocol, to gain access to information, which would allow an appropriate metablock size to be determined.
Where data is relocated, adaptive metablock size may be based on the number of logical groups that contain relocated data. Where control data is stored in adaptive metablocks the adaptive metablock size may be fixed according to the type of data to be stored. Adaptive metablock size may be determined based on balancing increased parallelism obtained with large adaptive metablocks with reduced garbage collection obtained with smaller adaptive metablocks. Once the number of erase blocks required is determined by the adaptive metablock manager, a request for that number of erase blocks is sent to the block allocation manager.
A block allocation manager selects erase blocks from separate planes of the memory array. The planes may be selected based on the number of available erase blocks in the plane. Where adaptive metablocks of various sizes are used, planes may be filled to different levels. Thus, some planes could become full while others still have available erase blocks. Should this happen, a plane of the array would be unavailable and parallelism would be limited accordingly. To prevent or defer this happening, a block allocation manager gives a low priority to planes containing a small number of available erase blocks and a high priority to planes containing a large number of available erase blocks when assigning erase blocks to form an adaptive metablock. Planes that are still busy from a previous operation may be given a low priority also. Planes having data for relocation may be given a high priority where data may be relocated within a plane in a more efficient manner than relocating from one plane to another. The block allocation manager selects available erase blocks from an allocation block list (ABL).
Erased blocks are managed separately for each plane of the array. When a plane is selected, any erase block from that plane may be chosen to form part of an adaptive metablock. Typically, erase blocks are chosen from the top of a list, while newly available erase blocks are added to the bottom of the list. Erase blocks are managed by a hierarchy of lists as shown in
The Allocation Block List (ABL) 1810 is a short list of erased block addresses from which erased blocks are selected to form metablocks. Thus, ABL 1810 is at the top of the hierarchy of lists. Within ABL 1810, separate fields are maintained for each plane of the memory array. Typically, ABL 1810 is maintained in a non-volatile memory such as controller RAM. However, a copy is maintained in the non-volatile memory also.
A copy of ABL 1810 is written to a Log 1813 every time an adaptive metablock is formed and the erased blocks used to form it are removed from ABL 1810. Thus, the copy of ABL 1810 in Log 1813 is regularly updated. When an erased block becomes available through an erase operation, it is added to ABL 1810 in the field corresponding to the plane containing the erase block. ABL 1810 may be restored after a loss of power by copying from Log 1813. However, the Log copy may not be up-to-date because of the addition of erased blocks to ABL 1810 since the previous copying to Log 1813. Such erased blocks are easily identified from other data structures. Specifically, Log 1813 contains records of allocated metablocks. Allocated metablocks are metablocks, or adaptive metablocks, in which data are currently being updated by the host. Thus, when power is first applied, the first sector of each erase block of the original metablock may be scanned to determine if the erase blocks of the original metablock have been erased. If an erase block has been erased, its address is added to the ABL. Address data is maintained in Log 1813 as a starting logical group address concatenated with the format shown in
ABL 1810 may be initialized by moving a predefined number of block addresses from an Erased Block List (EBL) 1811. Each field of the ABL may be initialized by moving addresses from the corresponding EBL field. For example, ABL fields may be filled to half their capacity. When a block is required for allocation to a metablock, the first block in the relevant ABL field is used and its address is removed from the ABL. When a block is erased during garbage collection, it is added to the end of the relevant ABL field.
ABL 1810 may also be refilled with erased block addresses from EBL 1811. This may be necessary where. ABL 1810 is empty. Erased block addresses may be exchanged between ABL 1810 and EBL 1811 when a field of ABL 1810 is full or empty. Exchange may be done for just one field (or plane of the array) or for all fields. The exchange may include topping up ABL 1810 or may include a full exchange of all the entries in ABL 1810. An exchange may be triggered by a field becoming full or empty or may be triggered by another event or done on a periodic basis.
EBL 1811 is generally maintained in a sector that is held in non-volatile memory. It contains a list of erased blocks with separate fields for each plane of the array. It is in the same format as ABL 1810 and thus, entries may easily be exchanged between EBL 1811 and ABL 1810. Because EBL 1811 is maintained as a single sector in non-volatile memory, it may be rapidly accessed and updated thus facilitating exchange between EBL 1811 and ABL 1810. The exchange of addresses between EBL and ABL may occur when the ABL is full or empty. Alternatively, the exchange may occur more frequently to avoid heavy usage of particular locations in the memory array. The addresses in EBL 1811 may be exchanged with ABL 1810 and also with Plane Block Lists.
An EBL sector may be maintained in an EBL block containing only EBL sectors.
A Plane Block List (PBL) such as PBL 1812 is maintained in non-volatile memory for each plane of the array. PBL 1812 is a list of erase blocks in a particular plane of the memory array. Erase blocks that are listed in either ABL 1810 or EBL 1811. are not listed in PBL 1812. PBL 1812 may occupy one sector, though the sector need not be full. Typically, PBLs are grouped together in a PBL block or PBL blocks. A PBL block is a dedicated block containing only PBL sectors. When information in a PBL sector is changed an updated version is written to the next position in the PBL block. The old sector is marked as obsolete. Only one valid PBL sector exists in a particular PBL block for a particular plane. However, two or more valid PBL sectors may exist for a particular plane if the PBL sectors are in different PBL blocks. A PBL sector has two fields, a set of entries that define the locations of erase blocks and a sector index that lists the positions of all valid PBL sectors within the. PBL block. The entries defining locations of erase blocks are not necessarily in any particular order. The order of entries may be the result of exchange with the corresponding EBL field. Only the index of the last written PBL sector is valid. In a partially written memory, there are a lot of erased blocks and thus a lot of PBL sectors requiring a lot of PBL blocks. However, as the memory is filled, the number of erased blocks diminishes and the number of PBL blocks needed diminishes. In a logically full memory system, there may be no PBL blocks. The exchange of addresses between PBL 1812 and EBL is similar to that between EBL and ABL. The exchange may be unidirectional or bi-directional. Where multiple PBL blocks are used, one PBL block may be the active block used for exchanges. The active PBL block may be periodically changed. A field in EBL 1811 may be updated from a single PBL sector as a background operation.
At the top of the hierarchy of
Log 1813 is below WSL 1814. Log 1813 stores a cumulative list of adaptive metablocks allocated for storage of sectors listed in WSL 1814. Log 1813 also contains copies of all WSLs at the time it is updated. Log 1813 is updated whenever a metablock is allocated. Log 1813 may be contained in a Log sector within a Log block. When information in Log 1813 is changed, a new Log sector is written in the next available position in the Log block. The previous Log sector becomes obsolete and only the last written Log sector is valid. Below Log 1813 are the Temporary Group Address Table (TGAT) 1815 and Group Address Table (GAT) 1816. GAT 1816 is an address table stored in sectors in non-volatile memory containing a physical address for every logical group arranged sequentially in logical group address order. Thus, the nth entry in GAT relates to the logical group with logical group address n. The address data stored in GAT 1816 is in the format shown in
GAT sectors may be stored in a dedicated GAT block that has entries for a logically contiguous set of logical groups. A GAT block is divided into two partitions a GAT partition and a TGAT partition. The GAT partition contains an original entry for each logical group in the logical address range of the GAT block. The TGAT partition contains sectors having the same format as GAT sectors. TGAT sectors are used to update address data before updating the GAT. Periodically, the GAT partition in a block is rewritten to incorporate updates recorded in sectors in the TGAT partition. A TGAT sector temporarily replaces a corresponding sector in the GAT to update address information. TGAT sectors contain an index of valid TGAT sectors. This index is only valid in the last written TGAT sector. No such index is needed for GAT. A TGAT sector updates a GAT sector with address information from the Log associated with a WSL. The WSL and Log entries are then deleted.
The physical sector address of a sector of data having a particular logical address may be determined from lists 1814-1816. The WSLs are first read to determine if the sector has been recently written. If so, the physical sector address is found from the metablock address corresponding to the sector's position in the WSL. If the sector is not found in the WSLs, an index in a TGAT sector is read to determine if the sector has a TGAT entry. If so, the physical sector address is determined by reading the appropriate TGAT sector. If the sector is not listed in either WSLs or TGAT then the appropriate GAT sector is read to determine its physical location. Look-ahead caching of Log, TGAT and GAT entries in controller SRAM can be performed to reduce address translation time when data is written or read in sequential address order.
Boot block 1820 is a dedicated block containing boot sectors. When information in the boot sector is changed, a new boot sector is written. Only the last written boot sector is valid. Boot block 1820 has a fixed physical location and is identified by scanning during system initialization. Scanning may be necessary because the location of the boot block is fixed within a range rather than at a precise location. This is to allow for the possibility of bad erase blocks. The location of the boot block may be fixed within a narrow range so the scanning may be rapidly completed. The boot sector contains the location of block addresses 1821 and any other system configuration information that may be required. Thus, upon initialization, the data structures in
Certain data structures described above use dedicated blocks such as the EBL block, PBL block and GAT block. Such dedicated blocks may be a single erase block of the memory array or may be an adaptive metablock comprising multiple erase blocks. One advantage of using an adaptive metablock is that the size of the adaptive metablock used may be adjusted to the amount of data to be held. For example, where a memory has a large number of erased blocks, there may be a lot of PBL sectors and so a large PBL block might be suitable. When the memory array fills with data, the number of erased blocks is less, thus the number of PBL sectors is less and a smaller PBL block might be suitable.
Where adaptive metablocks of less than the maximum size are used for control data, the control data may be programmed in parallel with other data. Where data is sent from a host to be programmed to a memory array, such parallel programming may allow control data to be updated simultaneously with the programming of host data. Thus, there is no interruption to the programming of host data while the control data is updated, though there may be a reduction in programming speed because of reduced parallelism available for the host data programming. Thus, the examples of parallel programming shown in
Although the invention has been described with respect to various exemplary embodiments, it will be understood that the invention is entitled to protection within the full scope of the appended claims.
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|U.S. Classification||711/103, 711/E12.008, 711/150|
|International Classification||G06F12/02, G06F12/00, G06F12/08|
|Cooperative Classification||G06F2212/7208, G06F2212/7207, G06F12/0246|
|Jun 2, 2004||AS||Assignment|
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Owner name: SANDISK TECHNOLOGIES INC., TEXAS
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